(1) Industrial Field of Utilization
The present invention relates to a method for manufacturing a semiconductor device, and more particularly to a method for manufacturing a semiconductor device including a method for reforming an insulating film formed by a low temperature CVD method.
(2) Prior Art
There are a variety of methods for forming an insulating film by a Chemical Vapor Deposition (CVD) method, but a thermal CVD method and a plasma CVD method are mainly used in recent years. Now, still higher refinement and higher densification are demanded in a very Large Scale Integrated Circuit (VLSI) and in a DRAM in particular. Thus, it is undesirable to form an insulating film by the plasma CVD method because step coverage is inferior and impurities such as carbon (C) are contained in the formed insulating film. As a result, a thermal CVD method capable of forming a film at a low temperature without thermal influence upon other components has attracted attention for forming a refined and highly densified DRAM, but further improvement of the film quality is desired.
There have been those methods that are conducted under the following conditions as a thermal CVD methods capable of forming a film at a low temperature.
1 reaction gas is a mixed gas of mono-silane (SiH.sub.4)/oxygen (O.sub.2), and the temperature is from 350.degree. C. to 450.degree. C.
2 reaction gas is a mixed gas of organic silane (TEOS)/ozone (0.sub.3) , and temperature is from 350.degree. C. to 450.degree. C.
Since the forming temperature is low in both cases, it is possible to restrain thermal distortion, thus exerting small influence upon the circumference. Therefore, such CVD method is adopted in many cases to form a passivation film and the like on an Al interconnection layer.
In case of a film not over 1 .mu.m, however, the quality of an interlevel insulating film and the like exerts a big influence upon characteristics of a device, and the following problems have been newly encountered with an interlevel insulating film formed by the thermal CVD method. Namely, such problems that are related to reliability include:
1 lowering of dielectric breakdown strength,
2 increase of leakage current, and
3 corrosion of an Al film and the like
which problems are caused sometimes because of such as reasons
1 the density of the formed insulating film is low,
2 moisture (II.sub.2 O) and bonds such as Si--OH are contained in the formed insulating film, and
3 bonds such as Si--H are present.
In order to solve such problems, it is required:
1 to form a film at the highest possible temperature,
2 to form a film while reducing growth rate to the minimum,
3 to increase O.sub.3 concentration to the maximum in a method using TEOS, and
4 to anneal the film at a highest possible temperature after film formation. There are limits to such solutions. For example, it is not desirable to adjust the temperature to above 450.degree. C. from a viewpoint of diffusion of Al into a Si substrate, generation of hillocks and the like in case of forming an interlevel insulating film for covering a lower Al interconnection layer. Further, the growth rate cannot be reduced too low without sacrificing mass productivity.